Add options uid, gid, uname, and gname to docker.nix.
Setting these to e.g. 1000, 1000, "user", "user" will build an image
which runs and allows using Nix as that user.
When leveraging remote builders or cache in CI workloads, sometimes you need to configure nix to connect via SSH to a remote server.
It is the case for example when using nixbuild.net.
By including `openssh` package, CI should be able to reach remote builders when configured i.e. with environment variables.
nix profile will otherwise throw this error:
error: path '/nix/var/nix/profiles/default/manifest.nix' is not in the Nix store
That's not entirely true since manifest.nix is within a directory in
the nix store but nix profile seems to require the manifest.nix itself
to be a store path.
1. `target` is the wrong name, that is just for compilers per out
standard terminology. We just need to worry about "build" and "host".
2. We only need one `pkgs`. `pkgs.buildPackages` is how we get anything
we need at build time.
3. `crossSystem` is the name of a nixpkgs parameter that is actually an
attribute set, not a 2-part "cpu-os" string.
3. `pkgsCross` effectively evaluates Nixpkgs twice, which is
inefficient. It is just there for people poking around the CLI / REPL
(and I am skeptical even that is a good idea), and *not* what written
code should use, especially code that is merely parametric in the package set
it is given.
4. We don't need to memoize Nixpkgs here because we are only doing one
pkg set at a time (no `genAttrs`) so it's better to just delete all this
stuff. `flake.nix` instead would do something like that, with
`genAttrs` (though without `pkgsCross`), if and when we have hydra jobs
for cross builds.
